[enh] Add auto push to github https://github.com/duniter/cesium-plus-pod

.github/CONTRIBUTING.md

+This project is now tracked on our hosted gitlab server at:
+> https://git.duniter.org/clients/cesium-grp/cesium-plus-pod
+
+The current github repository is a simple clone taken up to date at each push on the main gitlab repository.
+
+All contributions should be performed on the main gitlab repository.
+
+Pull requests proposed on github would generate more work for the main contributors.
+
+Issues can be submitted on github. However, all issues created on github will be duplicated on gitlab manually and closed with a link to the gitlab issue.

.github/ISSUE_TEMPLATE.md

+Please note that all issues should be preferentially opened at
+https://git.duniter.org/clients/cesium-grp/cesium-plus-pod/issues/new?issue
+
+If you open issue here, it will be duplicated manually on our hosted gitlab and closed here with a link to our main repository.
+
+Thank you for your understanding.

.github/PULL_REQUEST_TEMPLATE.md

+This project is now tracked on our hosted gitlab server at:
+> https://git.duniter.org/clients/cesium-grp/cesium-plus-pod
+
+The current github repository is a simple clone taken up to date at each push on the main gitlab repository.
+
+All contributions should be performed on the main gitlab repository.
+
+Pull requests proposed on github would generate more work for the main contributors, therefore we would really appreciate if you can create instead merge requests on our hosted gitlab instance.
+
+Thank you for your understanding.

.github/github_disclaimer.md

+# Initial disclaimer
+This project is now tracked on our hosted gitlab server at:
+https://git.duniter.org/clients/cesium-grp/cesium-plus-pod
+
+The current github repository is a simple clone taken up to date at each push on the main gitlab repository.
+
+All contributions should be performed on the main gitlab repository.
+
+Pull requests proposed on github would generate more work for the main contributors.
+
+Issues can be submitted on github. However, all issues created on github will be duplicated on gitlab manually and closed with a link to the gitlab issue.
+
+
+# Original README.md

.gitlab-ci.yml

+stages:
+    - github-sync
+
+push_to_github:
+    stage: github-sync
+    variables:
+        GIT_STRATEGY: none
+    tags:
+        - github
+    script:
+        - rm -rf ./*
+        - rm -rf .git
+        - git clone --mirror $CI_REPOSITORY_URL .
+        - git remote add github $GITHUB_URL_AND_KEY
+        - git config --global user.email "contact@duniter.org"
+        - git config --global user.name "Duniter"
+        # Job would fail if we don't remove refs about pull requests
+        - bash -c "cat packed-refs | grep -v 'refs/pull' > packed-refs-new; echo 'Removed pull refs.'"
+        - mv packed-refs-new packed-refs
+        - bash -c "git push --force --mirror github 2>&1 | grep -v duniter-gitlab; echo $?"
+
+.enforce_readme:
+    stage: github-sync
+    variables:
+        GIT_STRATEGY: none
+    tags:
+      - github
+    script:
+      - rm -rf ./*
+      - rm -rf .git
+      - git clone $GITHUB_URL_AND_KEY .
+      - git config --global user.email "contact@duniter.org"
+      - git config --global user.name "Duniter"
+      - git checkout master
+      - cat .github/github_disclaimer.md > README.md.new
+      - cat README.md >> README.md.new
+      - mv README.md.new README.md
+      - git commit -am "Enforce github readme"
+      - git push origin master
+